1. Field of the Invention
The present invention relates to a recording apparatus provided with a recording head in which an energy conversion element is formed for converting an electrical energy into a printing energy for printing on a recording medium, and more particularly a recording apparatus provided with a recording head including a semiconductor substrate in which formed is a print energy generating element for generating a print energy. The printing on the recording medium includes not only an operation of printing a character but also an operation of a symbol, a pattern etc. other than the character.
2. Related Background Art
There is already known so-called bubble jet recording method, or an ink jet recording method which gives an energy such as heat to an ink thereby causing a state change involving a rapid volume change (generation of a bubble) in a liquid such as ink, discharging the ink from a discharge port by a force based on such state change and depositing the ink on a recording medium thereby achieving an image formation. A recording apparatus utilizing such bubble ink jet recording method is generally provided, as disclosed in U.S. Pat. No. 4,723,129, with a discharge port for discharging the ink, an ink flow path communicating with the discharge port, and a heat-generating resistor provided in the ink flow path and constituting energy generation means for ink discharge.
Such recording method has various advantages such as being capable of recording an image of a high quality with a low noise level, and, since the discharge ports for discharging ink can be arranged in a high density in a recording head for executing such recording method, also capable of obtaining a recorded image of a high resolution or even a color image with a compact apparatus. Therefore the bubble jet recording method is recently utilized in various office equipment such as a printer, a copying machine and a facsimile, and is being further utilized in industrial systems such as a textile dyeing apparatus.
The heat generating resistor for generating energy for ink discharge can be prepared by a semiconductor manufacturing process. Therefore, a conventional recording head utilizing the bubble jet technology has a configuration obtained by forming a heat generating resistor on an element substrate constituted by a silicon substrate, and adhering thereon a top plate which is composed of a resin such as polysulfone or glass and in which a groove is formed for forming the ink flow path.
Utilizing a fact that the element substrate is composed of a silicon substrate, there is also known a configuration in which not only the heat generating resistor is formed on the element substrate but also a driver for driving the heat generating resistor, a temperature sensor to be used in controlling the heat generating resistor according to the temperature of the head and a drive control unit therefore are formed on the element substrate (for example Japanese Patent Application Laid-Open No. 7-52387). A head including the driver, the temperature sensor, the drive control unit therefore etc. on the element substrate is already in commercial use, and is contributing to an improvement in the reliability of the recording head and a compactization of the apparatus.
FIG. 6 is a block diagram showing an example of the driver formed on the element substrate. A recording head includes 128 nozzles (discharge ports) and heat generating resistors corresponding thereto. The heat generating resistors are represented by seg1 to seg128. A terminal Vh is a common terminal which is common to 128 heat generating resistors. Vh is given a voltage of 20 to 35 V at the recording operation. A terminal Top(Rnk) is used for judging a rank of the recording head in a tank table to be explained later, and corrects a width or a height of a driving pulse for the heat generating resistor or a drive timing according to the value of an internal rank resistor 141, thereby achieving such control as to discharge an ink droplet of a same volume from any recording head. A terminal GNDH provides a reference potential for a heat generating resistor drive circuit 128. A terminal SUB is used for a sub heater 142. The sub heater 142 is used in case of elevating the temperature of the recording head. The sub heater 142 is provided in two units, at the left and right sides of the recording head.
HeatEN-A and HeatEN-B indicate enable signal terminals for driving the heat generating resistors. The HeatEN-A and HeatEN-B are made independently controllable. Terminals RESET, CLK-A, CLK-B and U/D are related to counters 144A, 144B for selecting a nozzle for data setting, in each block. Next to the counters 144 there is provided a decoder 145, and, next thereto there is provided a logic circuit which forms a logic product with a recording signal and which is connected to the heat generating resistors through a transistor array 147. RESET indicates a clear terminal for the counters 144. CLK-A and CLK-B indicate clock terminals for input to the counters 144A and 144B. U/D indicates a terminal for selecting an up/down state of the counters 144. In a reciprocating recording operation, the recording is executed by alternating the up state and the down state, for example an up state in the forward motion and a down state in the reverse motion. IDATA indicates a data input terminal, in which data are entered in synchronization with a data clock signal from a DCLK terminal, then guided through a serial-parallel conversion circuit 148 of 128 bits, and temporarily latched in a latch circuit of 128 bits. The RESET terminals also serves for resetting the latch circuit 149, and an LTCLK terminal is used for providing the latch circuit 149 with a latch signal.
A terminal VDD is a power supply voltage input terminal for a logic system, and applies a voltage of 5 V. A terminal GNDL provides a reference voltage of the logic system. A terminal DiA is a terminal for two diodes 150 serially connected to a terminal DiK. The two diodes 150 are positioned at the left and right sides of the recording head and are used for measuring an average temperature of the recording head.
As explained in the foregoing, the recording apparatus applies a heat pulse, which is a pulse-shaped voltage, across the heat generating resistors seg1 to seg128 of the recording head, thereby driving the heat generating resistors seg1 to seg128, whereby the heat generated by the heat generating resistors seg1 to seg128 induces a bubble generation in the nearby ink and the ink is discharged by a pressure of such bubble generation. Consequently, in the recording head of such recording apparatus, the discharge amount of the ink principally depends on the volume of the bubble generated in the ink. Since the volume of the generated bubble in the ink varies by the temperature of the ink in the vicinity of the heat generating resistors seg1 to seg128, a preheat pulse (first pulse voltage) which is a pulse of an energy level not causing ink discharge is applied prior to the application of a heat pulse (second pulse voltage) for ink discharge, and the surface temperature of the heat generating resistors seg1 to seg128 is regulated by the pulse width and the timing of such preheat pulse whereby discharged liquid droplets are made constant and the print quality is maintained.
FIG. 7 is a chart showing a change in the surface temperature of the heat generating resistors seg1 to seg128 at the ink discharge in a conventional recording apparatus, and a wave form of the pulse voltage applied to the heat generating resistors seg1 to seg128.
At a time t0 when the preheat pulse is entered into the heat generating resistors, the surface temperature T0 of the heat generating resistors seg1 to seg128 is same as the temperature of the recording head, namely an environmental temperature. Since the ink discharge amount varies depending on the temperature of the ink as explained in the foregoing, the recording apparatus is further equipped with control means for maintaining the environmental temperature T0 constant, utilizing the sub heater 142 and the aforementioned temperature sensor.
In response to the application of the preheat pulse to the heat generating resistors seg1 to seg128 at the time t0, the surface temperature of the heat generating resistors seg1 to seg128 which has been T0 same as the room temperature is elevated. When the application of the preheat pulse is terminated at a time t1, the surface temperature of the heat generating resistors seg1 to seg128 which has reached a temperature T1 starts to descend. Since the temperature T1 is lower than 300° C. which is a bubble generating temperature of the ink, no bubble generation takes place in the ink up to this point. When a heat pulse is applied to the heat generating resistors at a time t2, the surface temperature of the heat generating resistors seg1 to seg128 which has descended to a temperature T2 starts to rise again. At a time t3 when the surface temperature of the heat generating resistors seg1 to seg128 reaches T3 (=300° C.), a bubble generation takes place in the ink. Upon the bubble generation, the heat is no longer transmitted from the heat generating resistors seg1 to seg128 to the ink, so that the surface temperature of the heat generating resistors seg1 to seg128 rises rapidly. Such temperature reaches a peak value T4 at a time t4 when the application of the heat pulse to the heat generating resistors seg1 to seg128 is terminated. After the time t4 when the application of the heat pulse to the heat generating resistors seg1 to seg128 is terminated, since the thermal energy is no longer generated from the heat generating resistors seg1 to seg128, the surface temperature of the heat generating resistors seg1 to seg128 is rapidly lowered and returns to the original environmental temperature T0. A pulse width of the preheat pulse is represented by P1, a predetermined off time from the termination of the application of the preheat pulse to the start of the application of the heat pulse is represented by P2, and a pulse width of the heat pulse is represented by P3.
It is confirmed that the ink discharge amount varies significantly by the lengths of the pulse width P1 and the off time P2. FIG. 8 is a chart showing the relationship between the pulse width P1 and the ink discharge amount when the environmental temperature T0, the off time P2 and the pulse width P3 of the heat pulse are maintained constant.
In FIG. 8, curves a, b and c respectively represent the relation between the preheat pulse width P1 and the ink discharge amount in recording heads different in the structure or in the driving conditions. V0 indicates an ink discharge amount at P1=0 μs. In a recording head represented by the curve a, in response to an increase in the preheat pulse width P1, the ink discharge amount Vd increases with a linearity in a range of the pulse width P1 from 0 to P1LMT, but loses the linearity of the change in a range where the pulse width P1 is larger than P1LMT and reaches a saturated maximum at a pulse width P1MAX. A range up to the pulse width P1LMT, where the discharge amount Vd shows a linear change with respect to the change of the pulse width P1, is an effective range enabling an easy control of the discharge amount by the change of the pulse width P1. In case the pulse width is larger than P1MAX, the discharge amount Vd becomes smaller than VMAX. This is due to a phenomenon that, when a preheat pulse of a pulse width within the aforementioned range is applied, a small bubble (a state immediately before a film boiling) is generated on the heat generating resistor and a next heat pulse is applied before such small bubble is extinguished whereby such small bubble disturbs the bubble generation by the heat pulse, thereby resulting in a smaller discharge amount. Such range is called a prebubbling range, in which a discharge amount control by means of the preheat pulse as explained later becomes difficult. In FIG. 8, a preheat pulse dependence coefficient Kp, defining the inclination of a linear line indicating the relationship between the discharge amount within a range P1=0 to P1LMT μs, is given by:Kp=ΔVdP/ΔP1(ng/μsec·dot) 
This coefficient Kp is independent from the temperature and is determined by the structure and the drive conditions of the recording head, and the physical properties of the ink. As explained in the foregoing, the curves b and c in FIG. 8 indicate the preheat pulse depending characteristics of other recording heads, and it will be understood that the discharge characteristics become different for different recording heads. Also the upper limit P1LMT of the preheat pulse width P1 is different for the different recording heads. As a reference, in the recording head and the ink represented by the curve a, Kp is 3.209 (ng/μsec·dot).
FIG. 9 is a chart showing a relationship between the off time P2 and the ink discharge amount when the environmental temperature T0, the preheat pulse width P1 and the heat pulse width P3 are maintained constant. As shown in FIG. 9, within a range up to a value P2MAX which is determined by the head structure and the physical properties of the ink, the ink discharge amount Vd increases with an increase in the off time P2. This is because, with a longer off time P2, the energy given to the ink at the application of the heat pulse can diffuse more sufficiently in the ink. When the off time exceeds P2MAX, the ink discharge amount Vd decreases with an increase in the off time P2. This is because an excessively long off time P2 loses the effect of increasing the ink discharge amount by the application of the preheat pulse.
On the other hand, with respect to an energy required for causing bubble generation in the liquid in contact with the aforementioned heat generating resistors seg1 to seg128, such energy is given by a product of an energy required for a unit area of the heat generating resistors seg1 to seg128 and the area of the heat generating resistors seg1 to seg128. Consequently, in order to obtain a desired ink discharge amount, there are required, as conditions for generating the energy required for discharging the desired amount of ink, in addition to a required area of the heat generating resistors seg1 to seg128, a voltage to be applied across the heat generating resistors seg1 to seg128, a current in the heat generating resistors seg1 to seg128 and a time thereof.
However, in a configuration where the heat generating resistors seg1 to seg128 are formed on an element substrate, the film thickness of the heat generating resistors seg1 to seg128 shows a fluctuation among produced recording head because of the manufacturing process of the recording head, so that the resistance of the heat generating resistors seg1 to seg128 shows a fluctuation of about ±20% among the recording heads, taking, as a standard, a resistance when the heat generating resistors have a film thickness as designed. Stated differently, even in case of producing plural recording heads of a same type, the resistance of the heat generating resistors seg1 to seg128 fluctuates from head to head. Consequently, though the voltage applied to the heat generating resistors seg1 to seg128 can be made substantially constant by a power supply in a main body of the printing apparatus, the current flowing in the heat generating resistors seg1 to seg128 becomes different from head to head because of the fluctuation in the film thickness of the heat generating resistors seg1 to seg128. In case the current becomes smaller by a resistance of the heat generating resistors seg1 to seg128 larger than the standard value, the volume of the bubble generated in the ink becomes smaller because of a deficiency in the charged energy. On the other hand, in case the current becomes larger by a resistance of the heat generating resistors seg1 to seg128 smaller than the standard value, the volume of the bubble generated in the ink becomes larger because of an excess in the charged energy. Stated differently, in case the resistance of the heat generating resistors seg1 to seg128 fluctuates among the recording heads, an error is generated in the ink discharge amount in each recording head, even if a same pulse voltage is given for a same time to all the recording heads.
For solving such drawback, there is conventionally adopted a method, utilizing the characteristics of the ink discharge amount shown in FIGS. 8 and 9, of changing the set value of the preheat pulse width P1, the off time P2 and the heat pulse width P3 for each head, according to the resistance of the heat generating resistors seg1 to seg128.
As shown in FIG. 6, the element substrate is provided, separately from the heat generating resistors seg1 to seg128, with a rank resistor 141 for recognizing the resistance of the heat generating resistors seg1 to seg128. Conventionally, in adjusting the preheat pulse width P1, the off time P2 and the heat pulse width P3, there is adopted a method of measuring the resistance of the rank resistor 141, referring to a rank table to be explained later based on the measured value and setting values P1, P2 and P3 corresponding to the rank of the resistance of the rank resistor 141 as the preheat pulse width P1, the off time P2 and the heat pulse width P3 of such recording head.
FIG. 10 shows a rank table to be used for determining the preheat pulse width P1, the off time P2 and the heat pulse width P3. This rank table is for a recording head in which the rank resistor has a standard resistance of about 1044 to 1057 Ω and a fluctuation within a range of 884 to 1228 Ω. In this rank table, the resistance of 884 to 1228 Ω of the rank resistor is divided into 24 ranks, and a preheat pulse width P1, an off time P2 and a heat pulse width P3 are set for each rank. As shown in the rank table in FIG. 10, the off time P2 is fixed in all the ranks, in order to obtain, in all the ranks, a constant diffusion state in the ink of the thermal energy given by the application of the preheat pulse. As the resistance of the rank resistor 141 becomes smaller, the current in the heat generating resistors seg1 to seg128 will become larger, so that the preheat pulse width P1 and the heat pulse width P3 are selected shorter, and, as the resistance of the rank resistor 141 becomes larger, the current in the heat generating resistors seg1 to seg128 will become smaller, so that the preheat pulse width P1 and the heat pulse width P3 are selected longer. The preheat pulse width P1 in the rank table is so selected as to reach the aforementioned temperature T1 in each recording head.
Recently, in the recording apparatus utilizing the aforementioned recording head, there is shown a tendency of reducing the total pulse width of the preheat pulse and the heat pulse, in order to achieve a higher speed in the recording. A reduction in the pulse width of these pulses provides an advantage of reducing the amount of heat radiation from the heat generating resistors and also an advantage that the stability of the discharge state is increased because the ink temperature rises uniformly. The total pulse width of the preheat pulse and the heat pulse has conventionally been 3.5 to 5.5 μs, but is recently as short as 3 μs or less.
FIG. 11 is a chart showing a relationship between the total pulse width of the preheat pulse and the heat pulse, and the ink discharge amount. It will be seen that the ink discharge amount is almost constant within a range of 7.5 to 8.3 ng when the total pulse width of the preheat pulse and the heat pulse is within a range of 3.5 to 5.5 μs, but the ink discharge amount decreases rapidly with a reduction in the pulse width, in case the total pulse width is 3 μs or less.
In the rank table shown in FIG. 10, the preheat pulse width P1, the off time P2 and the heat pulse width P3 are selected on a condition that the ink discharge amount is substantially constant irrespective of the total pulse width of the preheat pulse and the heat pulse selected on the rank table, and, in such a case that the ink discharge amount shows a rapid change by the total pulse width of the preheat pulse and the heat pulse, the ink discharge amount will eventually change even if the values in the rank table are merely applied.
In the conventional recording apparatus, as explained in the foregoing, the ink discharge amount shows a rapid decrease as the total pulse width of the preheat pulse and the heat pulse is decreased.
However, the tank table, to be referred to for determining the preheat pulse width, the off time and the heat pulse width for each produced recording head, is prepared on a condition that the ink discharge amount is almost constant irrespective of the total pulse width of the preheat pulse and the heat pulse. Consequently, even if the preheat pulse width and the heat pulse width are set at the values given in the rank table, the ink discharge amount shows a rapid decrease in case the total pulse width of the preheat pulse and the heat pulse is short, whereby encountered is a drawback that the ink discharge amount shows fluctuation among the recording heads.